gro: optimize skb_gro_postpull_rcsum()

We can leverage third argument to csum_partial():

  X = csum_sub(X, csum_partial(start, len, 0));

  -->

  X = csum_add(X, ~csum_partial(start, len, 0));

  -->

  X = ~csum_partial(start, len, ~X);

This removes one add/adc pair and its dependency against the carry flag.
Signed-off-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

include/net/gro.h

@@ -173,8 +173,8 @@ static inline void skb_gro_postpull_rcsum(struct sk_buff *skb,
 					const void *start, unsigned int len)
 {
 	if (NAPI_GRO_CB(skb)->csum_valid)
-		NAPI_GRO_CB(skb)->csum = csum_sub(NAPI_GRO_CB(skb)->csum,
-						  csum_partial(start, len, 0));
+		NAPI_GRO_CB(skb)->csum = ~csum_partial(start, len,
+						       ~NAPI_GRO_CB(skb)->csum);
 }
 
 /* GRO checksum functions. These are logical equivalents of the normal